KIEV — One Ukrainian serviceman has been killed and eight wounded in fresh separatist attacks, the military said in Kiev on Thursday, as the OSCE security watchdog warned of a "worrisome" spread of violence in eastern Ukraine.

Fighting has lessened significantly since a cease-fire between Ukrainian forces and Russian-backed rebels was declared in mid-February, but both sides accuse the other of violations and casualties are reported almost daily.

"Fighting has not died down along a broad stretch of the frontline from Krasnogorovka to Svitlodarsk," Ukrainian military spokesman Oleksander Motuzyanyk said, referring to government-controlled villages to the west and northeast of rebel-held city Donetsk.

"The enemy is actively using heavy weapons. … The area of fighting is expanding," he said.

The assessment was backed up by comments from Alexander Hug, deputy chief of the Organization for Security and Cooperation in Europe's monitoring mission in Ukraine.

"Worrisome is that the geographical scope of the conflict seems to be spreading," he said in his weekly report on the situation in the east.

The rebellion by Russian-backed separatists in Ukraine's industrialized east has created the biggest crisis in relations between Russia and the West since the end of the Cold War.

The West and Kiev say there is incontrovertible proof that Russia is providing men and weapons to the separatists — something Moscow denies.

Ukraine this week seized on the capture of two prisoners it said were Russian soldiers who had killed Ukrainian soldiers as further proof of direct Russian involvement in the conflict.

On Thursday, the OSCE said it had visited the men and reported that both said they had been to Ukraine "on missions" from Russia before.

"They claimed that they were on a reconnaissance mission. They were armed but had no orders to attack. … One of them stressed repeatedly that there were no Russian troops involved in fighting in Ukraine," the OSCE said in a statement.

Under the terms of February's cease-fire agreement, brokered by Ukraine, Russia, Germany and France to end the conflict which has killed over 6,200 since last April, both sides should have withdrawn large-calibre weapons and tanks from the front line.

The OSCE mission monitors weapons-holding areas in territory controlled by Kiev and by the rebels to make sure both sides are sticking to the terms of the truce.

This week, "in many of them, previously recorded weapons are now missing despite claims that withdrawal of heavy weapons was complete. The mission also observed movement or presence of weapons on both sides of the contact line," Hug said.

Meanwhile, senior separatist commander Eduard Basurin accused government troops of wounding two rebel fighters and two civilians in attacks on rebel territory in the past 24 hours, separatist press service DAN reported.